1. Field of the Invention
The present invention relates generally to automatic transmissions and clutch control units for vehicles and, more particularly, to an automatic transmission with a sensor fault detector able to detect a fault of a sensor which detects the rpm of an engine.
2. Description of the Prior Art
FIG. 9 shows a conventional an automatic transmission including a clutch control unit. A well known parallel axis gear type transmission 1 is controlled by a transmission actuator 2 composed of a select actuator 2a and a shift actuator 2b. These actuators 2a and 2b are driven by a hydraulic drive unit 3 which consists of a tank 3a, a pump 3b, an accumulator 3c, and a hydraulic pressure switching electromagnetic valve (not shown) to control the driving gear of the transmission 1 via a pair of piston rods 2c and 2d. This control is made possible by the central processing unit (CPU) of a transmission drive unit 2e. The drive unit 2e is controlled through serial communications by a main control unit 4 which has a CPU, a read only memory (ROM), and a random access memory (RAM). A pair of potentiometers 2f and 2g sense the positions of the piston rods 2c and 2d. The position signals from the potentiometers 2f and 2g are fed back to the drive unit 2e to control the gear position so that the position signals match the target signals from the main control unit 4. An input shaft sensor 5a senses the rpm v of an input shaft 1a of the transmission. A vehicle speed sensor 5b senses the vehicle speed or rpm V' of an output shaft 1b of the transmission 1. The outputs of these sensors 5a and 5b are fed to the main control unit 4.
A clutch 6 is interlocked with the piston rod 7a of a clutch actuator 7 for control. The hydraulic pressure is supplied to the clutch actuator 7 from the drive unit 3, and the clutch 6 is feedback controlled by the CPU of a clutch drive unit 7b so that the position of the piston rod 7a which is sensed by a potentiometer 7c matches the target signal from the main control unit 4. The clutch actuator 7 and the clutch drive unit 7b constitute a clutch control unit 70. The clutch drive unit 7b is controlled by the main control unit 4 through serial communications.
An engine 8 has a control unit 8a which is controlled by the main control unit 4. An engine rpm sensor 9 senses the rpm V of an output shaft 8b of the engine 8. The revolutions of the engine 8 are transmitted to an axle 10 via the clutch 6 and the transmission 1. The output of the engine rpm sensor 9 is fed to the main control unit 4 for performing various controls such as a fuel supply control. More signals are fed to the main control unit 4 from an accelerator pedal sensor 11, a brake pedal sensor 12, an exhaust brake switch 14, and a selector 15 to control the transmission drive unit 2e, the clutch drive unit 7b, the engine control unit 8a , a display panel unit 16, and a control unit 17a for controlling a gear position display panel 17. Consequently, the main control unit 4 controls the clutch 6 and the transmission 1 according to the amount of depression of the accelerator pedal, the outputs of the respective sensors, and the positions of the selector 15 and gears of the transmission 1. The gear position is controlled by the select actuator 2a and the shift actuator 2b according to the set position of the selector 15. When the selector 15 is set in the position "1", "2", "3", or "R", the gear is controlled into the first, second, third, or reverse gear position. When the selector 15 is set in the automatic transmission position "D4" or "D5" , the gear is controlled automatically from the first to the fourth gear position or from the second to the fifth gear position according to the gear transmission map based on the amount of depression of the accelerator pedal and the vehicle speed.
Upon the above transmission control, the clutch 6 is automatically disengaged and then engaged by the main control unit 4 via the clutch actuator 7 and the clutch drive unit 7b. This control is made according to the clutch engagement characteristics obtained from learning activities made under predetermined conditions. Such learning activities are shown in FIG. 10. A learning starting point S is stored in the memory of the main control unit 4. When preconditions under which the engine is revolving, the gear is set in the neutral, and the vehicle is at rest, are satisfied, the clutch 6 is moved toward engagement from the learning starting point S to determine, as a learning point, a half clutch point F from the rpm v of the input shaft sensor 5a. Then, an engagement starting point Q and an engagement completion point P are determined by taking predetermined offset distances 1.sub.1 and 1.sub.2 toward disengagement and engagement from the half clutch point F, thereby making a clutch engagement characteristic curve which includes the half clutch point F, the clutch engagement completion point P, and the engagement starting point Q. The clutch engagement characteristic curve M is stored in the memory. In the next learning activity, the above engagement starting point Q is taken as a learning starting point S to perform a similar learning activity. By controlling the clutch 6 according to the clutch engagement characteristic curve thus obtained, it is possible to minimize the movement of the clutch 6, thereby not only speeding up the gear shift but also reducing the wear and tear of the clutch 6.
While only the half clutch point is determined by learning in the above example, two points; the engagement completion point and the engagement starting point may be determined by learning as described in Japanese Patent Application Kokai No. 60-34525.
In FIG. 9, a starter switch 18 is connected at one end to a battery and at the other end to a starter relay 19 which consists of a relay coil 19a and a relay contact 19b. The relay coil 19a is connected to the main control unit 4 via a starter drive line 19c which is turned on under predetermined conditions. The relay 19b is connected to a starter motor 20 for starting the engine 8. The juncture between the starter switch 18 and the starter relay 19 is connected to the main control unit 4 via a starter signal line 19d. An emergency switch 21 permits the starter to start unconditionally in the case of an emergency such as an engine failure on a railroad crossing.
The main control unit 4 not only controls the clutch 6 and the transmission 1 based on various input signals but also turns on the starter drive line 19c (L level) to enable the starter switch 18 to operate when the transmission 1 is in the neutral position. Japanese Patent Applications Kokoku No. 61-57576 and Kokai No. 59-213936 disclose that if there is no output from the engine rpm sensor, with the starter switch turned on, it is determined that the sensor is faulty, and the necessary backup operation is performed.
As has been described above, conventionally, when there is no output from the engine rpm sensor with the starter switch turned on, it is determined that the sensor is faulty. However, there can be other causes, such as a breakdown of the starter drive line or starter motor or a dead battery, that disable the engine to start. The conventional units are unable to detect such disabling causes.
Also, because of electrically unstable conditions caused at the time of start of the starter motor, the control unit can make an incorrect determination that the engine rpm sensor is faulty.